The present invention relates to casing joints particularly of the kind in which a box member and a pin member are respectively provided with complementary conical or tapered threads and wherein each of such members has one conical and one planar abutment surfaces for mutual engagement of the respective other complementary such abutment surface.
In joints of such type, one or both abutment surfaces of one of the element as engaging the respective other, facing, complementary abutment surface of the other element provides for sealing of the joint. Interengagement of both surfaces may seal the ends of both members of the joint, which is quite desirable in case a problem exists in that differently dimensioned box and pin members establish unforseeably different seals in that manner. Generally, that aspect has been recognized. It has not been recognized, however, that different degrees of yielding of the box and pin member end portions modify the seals further and in quite an unforseeable manner as far as the interengagement of abutment surfaces is concerned.
It has been suggested to provide one of the members with an annular groove to establish a definite higher degree of yielding of that member's end. If that groove is disposed at that end of the thread adjacent to which there is a plane abutment surface, one reduces therewith the impediment that surface provides against further threading.
Generally speaking, one can say that the state of the art has lead towards a development, tending to equalize the sealing effect of dual (axially spaced) abutment surfaces of the two members as interengaging.
It has been neglected, however, to prevent too deep a penetration of a pin member into the box member, casing widening of the latter and cracking on account of tangential tension. Even if the box member did not crack on making up of the joint, tension crack corrosion was often the result. Also, wherever sealing depended on a bulge behind one of the abutment ends, the bulge was often squeezed to such an extent that the respective member could not be reused. Generally speaking, the prior art neglected to provide a proper balance between minimum conditions of threading effecting and completing a seal, and undue application of threading force that would damage the joint.
The following patents are representative examples of the state of the art in which such box and pin members each have a shoulder and an end face, whereby the end face of one is to sealingly engage the shoulder of the other, and/or wherein tapered faces adjacent to such shoulders and end faces provide such seals: U.S. Pat. Nos. 3,359,013; 2,111,196; 2,239,942; 2,258,066; 2,772,899.
None of these references suggests or discloses the present invention.